Ceramic materials and light-weight materials are typically not synonymous with one another. Light-weight implies a low density and high porosity. Lighter weight ceramics are made from a few different techniques that include: the use of burnout materials, reticulated foam, vacuum casting, and expanding clay.
Burnout materials are typically organic-based materials added to clay batch recipes that burn out upon firing to form porous ceramic. The burnout material is a solid particle that occupies space in the clay material. During the firing process, the burnout material converts to carbon and then disappears during off-gassing, which leaves a void the size of the original particle. There is a limit to the amount of porosity produced which is dictated by the proximity of clay particles to one another for integrity.
Reticulated foam materials typically involve taking an open-cell polymer-based reticulated foam and impregnating it with a clay slurry. The foam is wrung-out leaving only a coating of ceramic slurry within the foam. Upon firing, the polymer foam burns away leaving the ceramic exo-skeleton. The exo-skeleton is dense, but the bulk porosity is dictated by the reticulated-foam. There are limits on how small the pore size can be and foam ceramics are typically simple shapes such as plaques.
Vacuum casting involves mixing a watered down clay slurry with fibres and using a vacuum to remove the liquid content. This leaves the slurry and fibres in a felt-like mass that needs to be dried before firing. Firing locks in the structure and produces a lighter weight ceramic block. Shapes typically include large blocks or complex geometries.
Expanding clay materials are produced by firing certain types of naturally occurring clay in a rotating kiln. This process uses the natural organics in the mined clay as an expanding agent to expand to form porosity. The result is a spherical particle with porous core and dense shell. Spherical particles are the only shape that can be produced and the composition and therefore properties, of the fired article are governed by the composition of the mined clay.
Lightweight concrete uses aeration processes or aggregates like porous slag to lower the concrete density. Aeration involves pumping air into the concrete which traps the air as pores that retain their shape during curing thus making a lighter weight product. Porous slag is a bi-product of the steel making industry. Porous, smaller particles are separated from the slag and these are blended into the concrete mix. Slag particles are somewhat less dense than concrete, 25% weight reduction being common for the lightweight concrete industry.
Improvements are possible in processes for manufacturing lightweight ceramic materials and in articles of manufacture made using such materials.